High temperature fuel cell generators employing interconnected, tubular fuel cells, with solid electrolytes, are taught by A. O. Isenberg, in U.S. Pat. No. 4,395,468. Support tube, fuel electrode, air electrode, solid electrolyte and interconnection configurations for individual fuel cells, are taught by A. O. Isenberg, in U.S. Ser. No. 323,641, filed on Nov. 20, 1981. Usually, a porous support tube of sintered, calcia stabilized zerconia powder, approximately 1 millimeter to 2 millimeters thick, having a 12 millimeter to 13 millimeter outside diameter, has an air electrode deposited on it. The air electrode is about 50 microns to 500 microns thick (0.05 millimeter to 0.5 millimeter) and may be made of, for example, LaMnO.sub.3, CaMnO.sub.3, LaNiO.sub.3, LaCoO.sub.3, LaCrO.sub.3, etc. Surrounding the outer periphery of the air electrode is a layer of gas-tight solid electrolyte, usually yttria stabilized zirconia, approximately 1 micron to 100 microns (0.001 millimeter to 0.1 millimeter) thick. A selected radial segment of the air electrode is covered by an interconnect material. The interconnect material may be made of a lanthanum chromite film, of approximately 50 microns (0.5 millimeter) thickness. Substantially surrounding the solid electrolyte is a porous fuel electrode, generally a nickel zirconia cermet anode, having a thickness of about 50 microns (0.05 millimeter). Thus, the support is usually from 4 to 20 times thicker than the other components of the cell.
The usual support tube consists of 15 mole percent calcia stabilized zirconia powder particles extruded into tubes, dried, and sintered at about 1650.degree. C. for 4 hours. Cellulose is used as a pore former and is burned off during sintering. The powder particles used are of general spherical form and are usually 4 microns to 20 microns average particle size. The primary function of the support tube used in high temperature solid oxide electrolyte fuel cells is to support the other fuel cell elements while allowing for adequate flow of gases, especially oxygen, through its structure. In order to pass oxygen through its structure a high level of porosity must be incorporated into the support tube during processing. It is well known that the strength of ceramics is proportional to e.sup.-bP, where b is an empirical constant in the range of 4 to 7 and P is the pore fraction. Therefore, as the porosity is increased strength decreases exponentially.
In order to optimize the performance of the support tube, a trade-off between mechanical strength and gas diffusion coefficients must be struck by varying tube porosity. Experience has shown that satisfactory cell operation and life can be realized using support tubes which have 30% porosity, 6750 psi strength and room temperature oxygen-diffusion coefficients of 0.010 cm.sup.2 /sec to 0.012 cm.sup.2 /sec. While these values are satisfactory, higher strengths and/or diffusion coefficients are desired. It would also be especially beneficial if lightweight, high strength support tubes could be developed, since they would allow a thinner support thickness, reducing the oxygen path to the air electrode, and reducing the size and weight of the fuel cell.